Roll-up doors are used in commercial or industrial establishments to separate different areas of a building, or to separate the inside of a building from the exterior. For example, roll-up doors may be used to separate two areas or zones which have different temperature or humidity conditions, or to provide noise control between two areas. A roll-up door has the advantage of being capable of being moved rapidly between the open and closed positions, and when in the open position, the door panel is wound on a drum located above the doorway, so that the door panel will not obstruct the doorway.
U.S. Pat. No. 3,878,879 describes a roll-up door, in which a roller is mounted above the doorway and a flexible door panel is wound on the roller. A winding drum is secured to each end of the roller and one end of a cable is secured to each winding drum. Each cable passes over a series of fixed pulleys and the opposite end of each cable is connected to the bottom of the door panel. To counterbalance the weight of the door panel, a weight is secured to each cable intermediate its ends.
In accordance with the construction of the aforementioned patent, trolleys are connected to the side edges of the door panel and ride in guideways on the frame as the door is wound and unwound on the roller, and the weight of the door panel is balanced by the proper selection of the diameter relation between the roller and the windup drum.
The roll-up door of U.S. Pat. No. 3,878,879 has certain disadvantages. If additional tensioning is required for the door panel when in the closed position to prevent wind deflection, it is necessary to add additional counterweight. However, added counterweight cannot be easily implemented and requires increased driving force to move the door panel to the closed position.